In the fields of deodorization, exhaust gas treatment and the like, various adsorbent materials have so far been developed. Activated carbon is one of the representative examples of them and it has been used widely in various industries for the purpose of air cleaning, desulfurization, denitrification, or removal of harmful substances by making use of its excellent adsorption performance. In recent years, a demand for nitrogen has been increasing, for example, in the semiconductor manufacturing process and the like. Such nitrogen is produced from the air by using molecular sieving carbon according to the pressure swing adsorption process or temperature swing adsorption process. Molecular sieving carbon is also used for separation and purification of various gases such as purification of hydrogen from a cracked methanol gas.
When a mixture of gases is separated according to the pressure swing adsorption process or temperature swing adsorption process, it is the common practice to separate it based on a difference between the gases in equilibrium adsorption amount or rate of adsorption to molecular sieving carbon or zeolite used as a separation adsorbent material. When the mixture of gases is separated based on a difference in equilibrium adsorption amount, conventional adsorbent materials cannot selectively adsorb thereto only a gas to be removed and a separation coefficient decreases, making it inevitable to increase the size of an apparatus used therefor. When the mixture of gases is separated into individual gases based on a difference in rate of adsorption, on the other hand, only a gas to be removed can be adsorbed, though depending on the kind of the gas. It is necessary, however, to alternately carry out adsorption and desorption and also in this case, the apparatus used therefor should have an increased size.
On the other hand, there has also been developed, as an adsorbent material providing superior adsorption performance, a coordination polymer undergoing a change in dynamic structure when exposed to external stimulation (refer to Non-patent Documents 1 and 2). When this novel coordination polymer undergoing a change in dynamic structure is used as a gas adsorbent material, it does not adsorb a gas until a predetermined pressure but it starts gas adsorption at a pressure exceeding the predetermined pressure. In addition, a phenomenon is observed in which an adsorption starting pressure differs depending on the nature of the gas.
Application of these phenomena to adsorbent materials used in a gas separation apparatus employing a pressure swing adsorption system enables very efficient gas separation. It can also decrease time required for pressure change, contributing to energy saving. Further, it can contribute to a size reduction of the gas separation apparatus, making it possible to increase competitiveness in terms of cost when a high-purity gas is put on the market as a product. To say nothing of it, even if the high purity gas is used in the own plant, the cost paid for the equipment requiring a high purity gas can be reduced, resulting in a reduction effect of a manufacturing cost of the final product.
There are known, as a using example of a coordination polymer undergoing a change in dynamic structure as a storage material or a separation material, (1) a metal complex having an interdigitated framework (refer to Patent Document 1), (2) a metal complex having a two-dimensional square-grid framework (refer to Patent Documents 2, 3, and 4), and (3) a metal complex having an interpenetrated framework (refer to Patent Document 5).
At present, however, it is requested to reduce the using amount of an adsorbent material in order to reduce the size of an apparatus, thereby achieving cost reduction. Although a further increase in the adsorption capacity is requested to achieve it, none of these related arts refers to an increase in the adsorption capacity. As a result of measurement at 273 K of an equilibrium adsorption amount of carbon dioxide to the metal complexes disclosed in Patent Document 1 to 4, it is about 80 mL (STP)/g and the amount of the metal complex described in Patent Document 5 is less than about 100 mL (STP)/g at most.
It is on the other hand known that some of the metal complexes having a two-dimensional square-grid framework have a large adsorption capacity (refer to Patent Documents 6, 7, and 8). These complexes, however, contain a tetrafluoroborate ion or trifluoromethanesulfonate ion and corrosion of a reaction container used in their manufacture or treatment of waste liquids upon manufacture poses a problem. They are therefore not suited for industrial manufacture. In addition, none of these coordination polymers starts adsorption of a gas until it exceeds a predetermined pressure so that when the partial pressure of a gas to be removed by adsorption from a mixture of gases is below the predetermined pressure, adsorption does not occur. An increase in the using amount of the metal complex is therefore inevitable to obtain a high purity gas and under such a situation, it is difficult to reduce the size of the apparatus.